Unlike cpi, the deflator has no fixed basket. Its weights are whatever the economy actually produced this quarter, and it excludes imports while including investment, government and export prices.
That makes it the broadest inflation measure but also the least timely and the most heavily revised. It is most useful for converting between nominal and real aggregates and for international comparisons where consumption baskets differ.
Example: nominal gdp grows 5.8% over a year while real GDP grows 2.4%. The implied deflator is 1.058 / 1.024 minus one, about 3.3%, which is the economy-wide inflation rate that reconciles the two.
Related: gdp, real-gdp, cpi, pce-price-index, nominal-yield